Choosing which version of Windows 7 to install
From Mac Guides
For most users and modern (2008+) Macs, the default answer should be Windows 7 Home Premium 64 bit edition.
Why might I pick the 32 bit version?
In certain cases the 32 bit version may be preferable.
The minimum system requirement for the 64 bit version is 2 GB. So if you have 2 GB or less of RAM or intend to use Windows 7 in a Virtual Machine with 2 GB or less of RAM allocated to the VM you may want to choose the 32 bit version (The 64 bit version uses more RAM to enable Windows Side By Side which allows it to run 32 bit applications).
Another reason you may want to choose 32 bit is if all of the applications you want to run are 32 bit only.
Yet another reason to consider the 32 bit version is that your Mac does not officially support the 64 bit version. See the Apple support article here: http://support.apple.com/kb/ht1846
Which edition of Windows?
The features that differentiate Professional/Ultimate from Home Premium are described here: http://windows.microsoft.com/en-US/windows7/products/compare
Can I use the OEM/System Builder's edition?
While it can work, it should be noted that OEM is not licensed to end users. http://www.microsoft.com/oem/en/licensing/sblicensing/pages/licensing_for_hobbyists.aspx describes the details of the license and restrictions placed on OEM editions that are not placed on retail versions.